The present invention relates to the structure of an upper hook provided with a hole and used in sewing machines for making overedge stitches, in particular sewing machines which are used in industry for making whipstitch No. 504 of the Federal Standard Catalog, on medium-lightweight fabrics.
As is known, sewing machines of the industrial type are able to form the so-called overedge stitch using three basic elements: a needle, an upper hook and a lower hook.
The needle and lower hook are always provided with thread, whereas the upper hook may or may not be provided with thread, depending on the type of stitch to be formed: in particular, it must be provided with thread in order to form the said overedge stitch No. 504 dealt with in the present invention.
The overedge stitch with three threads (that of the needle, that of the upper hook and that of the lower hook) is formed on the edges of a fabric and consists of various operating phases, known per se, in which the lower hook wedges itself between the needle and the needle thread, the upper hook wedges itself between the lower hook and the thread of the lower hook, and the needle wedges itself between the upper hook and the thread of the upper hook, obviously, before wedging itself between the said lower hook and the threads coming from the stitch previously formed. During these said operations, the degree of tensioning of the various threads is of vital importance not only for forming stitches with the threads themselves correctly arranged, as regards tensioning, but also so as to prevent irregularities and even the loss of stitches.
It has been shown in tests that thick and heavy fabrics make it easier, obviously within certain limits, to control and tension correctly the threads which form the stitches, since the fabrics themselves, on account of their greater thickness, firmly hold the threads and prevent any irregular tensioning of the latter, it not being possible to deform the said fabrics easily. On the other hand, very thin and lightweight fabrics can be easily deformed and, as a result, loose stitches can easily occur in the vicinity of tight and tensioned stitches, thus giving rise to undesired puckering of the fabric.
In practice, lightweight fabrics leave the threads substantially free and the latter are able to arrange themselves, in a random manner, in a large number of positions.
Another consequence of this situation is that it is practically impossible to form the overedge stitch using a very small pitch, since, in this case, the random fluctuations of the threads become too great and the danger arises of stitches being missed.
In fact, it is known that sewing machines of the industrial type which make overedge stitches with three threads on lightweight fabrics, operate with a maximum number of 38 stitches per inch.
Oscillations of the said threads occur in a particularly marked manner in the vicinity of the upper hook. In fact, the latter initially penetrates between the lower hook and associated thread, separating the latter, then causes the said thread of the lower hook to run along its blade, and finally brings its own thread into the vicinity of the tip of the needle. However, it may happen that either the upper hook engages with the thread of the lower hook in an imprecise manner or that the thread itself does not flow freely and is not guided properly along the blade of the upper hook, thus also running the risk of coming into contact with the support components of the said upper hook and becoming entangled with them or even of breaking. Also, the needle may fail to insert itself correctly between the upper hook and associated thread.
In view of the above, the general aim of the present invention is to design the structure of an upper hook provided with a hole so that it is able to remedy substantially the abovementioned drawbacks.
Within the context of this general aim, it is an important object of the present invention to design a said upper hook which is able to control efficiently the positions of the threads with which it engages, even in the case of operations involving lightweight fabrics.
Another important object of the present invention is to design an upper hook which has a simple structure and can be easily produced by the industries in the sector.